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  <script>
    // ES6新增的数据结构：WeakMap\WeakSet\Set\Map
    let obj1 = {}
    let obj2 = {}
    let obj3 = {}
    let arr = [obj1,obj2,obj3]
    obj1 = null
    obj2 = null
    obj3 = null
  // 三个对象不会被销毁，arr仍引用
  // P648未完结
  </script>
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